With the increase in environmental problems, bioethanol produced using a carbon-neutral biomass has become noticed as a novel fuel. Heretofore, bioethanol has been produced mainly from raw materials that compete with foods such as starch, sugar, etc., and therefore the problem thereof that would result in the decrease in raw material supply for foods and in the increase in the cost of the material has been pointed out. Given the situation, at present, a technique for producing ethanol from a cellulosic biomass not competing with foods has come to attract rising attention.
The cellulosic biomass includes, for example, trunks and empty fruit bunches of palm, fibers and seeds of palm fruits, bagasse, rice straws, wheat straws, corn residues (corn stovers, corn combs, corn hulls), Jatropha seed coats and hulls, wood chips, switchgrass, Erianthus, energy crops, etc. These all contain cellulose and hemicellulose that can be converted into sugar, and lignin.
Lignin is contained in the solid residue that is separated from cellulose and hemicellulose in the stage where the above-mentioned raw material is saccharified/fermented in the production process for bioethanol. The residue may be utilized as fuel as it is. However, lignin degradation provides phenol derivatives, and therefore rather than using the residue as fuel, developing it into any other chemical industrial products could provide further higher added value.
Consequently, it is desired to develop a method for efficiently producing a lignin degradation product capable of being used as a raw material for the above-mentioned chemical industrial products.
Technology of lignin dissolution has been developed mainly in pulp production. For example, in a kraft pulp method, a chemical agent containing mainly sodium hydroxide (NaOH) and sodium sulfide (Na2S) is added and boiled at 150 to 160° C. or so. On the other hand, in a sulfide pulp method, a mixed liquid of an acidic sulfite salt and sulfurous acid is added and boiled at 130 to 145° to dissolve lignin in wood as a lignin sulfonate salt (for example, see PTL 1).
However, these methods each use a strong alkali or a strong acid, and therefore as the material for the reactor and the fixings to be used therein, alkali-resistant or acid-resistant ones must be selected and the handleability thereof is not good. For example, PTL 1 describes “problems of relatively high equipment cost and contamination” in paragraph [0002]. In addition, the sulfide pulp method produces sulfonated lignin and is therefore limited in point of the use thereof.
Claim 1 in PTL 1 discloses a pulping step where a known pulp raw material such as a woody material or a crop waste, and an aqueous solvent containing from 50 to 90% of a high-boiling-point organic solvent that has a boiling point of from 150 to 250° C. and is at least soluble in water, are filled in a pressure-tight reactor in a liquid ratio of from 4 to 10, and processed therein at a temperature of from 180 to 230° C. In addition, in paragraph [0007], there are mentioned cyclic ethers and polyalcohols as the high-boiling-point solvent. Further, PTL 1 describes the advantage of recycling the high-boiling-point solvent as it is without separating from water. However, the high-boiling-point solvent is expensive and there still remains room for development in point of the degree of solubility in the solvent of the produced lignin.
Also proposed is a method for lignin separation by using an aqueous solvent that contains an organic solvent such as acetic acid, an alcohol solvent or the like, adding sodium hydroxide or a mineral acid as a catalyst, and so on (for example, see PTL 2, [0022]).
As in the above-mentioned PTL 1 and 2, methods for production of a lignin degradation product have been proposed, but from the viewpoint of obtaining a lignin degradation product as a raw material for high-value-added products in the field of chemical industry, from a carbon-neutral biomass in consideration of environmental problems, it is still desired to efficiently produce a lignin degradation product in an energy-saving manner. From this viewpoint, there is still room for improvement in the production methods for a lignin degradation product in the above-mentioned PTLs 1 and 2.
Also proposed is a method for producing a lignin degradation product from a lignocellulose biomass, using a mixed solvent prepared by adding from 5 to 20% by volume of water to an aliphatic alcohol having from 1 to 8 carbon atoms, under the supercritical or subcritical condition for the mixed solvent (see PTL 3).
However, in PTL 3, methanol that is actually used as the aliphatic alcohol in the mixed solvent does not separate under room temperature and the solvent is in a one-phase state. When the mixed solvent is in a one-phase state, the entire amount of the mixed solvent must be distilled for separating the lignin degradation product through distillation, and therefore a large amount of heat energy is consumed.
On the other hand, for example, in a case of a mixed solvent with an alcohol having a lower boiling point than that of water, the alcohol alone may be evaporated away from the mixed solvent and the lignin having dissolved in the alcohol could be precipitated and recovered. However, a part of lignin having dissolved in water could not be recovered. Consequently, according to the method disclosed in PTL 3, the heat energy loss is great and it is difficult to increase the recovery rate of lignin. In addition, the purity of the resultant lignin is low. Further, the production method in PTL 3 requires high-temperature and high-pressure production conditions, and therefore it is considered that the method would require expensive facilities and severe safety measures. In this way, also for the method in PTL 3, further improvement is desired for efficiently producing lignin in an energy-saving manner.
Also proposed is a method for obtaining a lignin degradation product by processing a lignocellulose biomass in a supercritical or subcritical 1-octanol (see PTL 4).
In the method described in PTL 4, a biomass raw material is extracted using a solution of an alcohol alone. The degradation rate of the lignin degradation product in a solution of an alcohol alone is not sufficient. In addition, it is difficult to separate the lignin having dissolved in the alcohol from cellulose and hemicellulose degradation products, and therefore it is difficult to increase the recovery rate of the lignin degradation product and the purity thereof. Further, for efficiently extracting the lignin degradation product, a large amount of an expensive alcohol must be used. In that manner, even in the method in PTL 4, further improvement is desired for efficiently producing a lignin degradation product in an energy-saving manner.
Also proposed is a method for recovering lignin from a lignocellulose substance by pulping with an alcohol having from 1 to 4 carbon atoms, water and NaOH at a temperature lower than 100° C. (see PTL 5, Claim 1). Claim 2 in PTL 5 describes the ratio of water/alcohol of from 10/90 to 90/10.
However, the method described in PTL 5 has the following problems. Specifically, the method requires a neutralization step for recovering lignin; the base recovery in the method is complicated and when the base could not be recovered, the production cost increases; lignin is often contaminated with base-derived impurities, and owing to the impurities, the lignin is denatured and the quality thereof lowers; and the method requires base-resistant reaction facilities.
Also disclosed is a method of bringing a biomass raw material into contact with a mixture of a water-immiscible organic solvent, an acid and a metal salt catalyst dissolved in an acidic aqueous solution, at a predetermined temperature and a predetermined pressure to thereby separate lignin and hemicellulose having dissolved in the solvent and the aqueous phase from each other, and to leave a pure cellulose as such (see PTL 6). Further, PTL 6 describes use of a higher alcohol as the solvent, saying that butanol and isoamyl alcohol are preferred as the higher alcohol and the ratio of the solvent to the acidic water is from 40/60 to 80/20.
However, the method described in PTL 6 has the following problems. Specifically, the method requires an acid neutralization step; since the catalyst is soluble in water, the catalyst recovery is complicated; the lignin is often contaminated with acid and catalyst-derived impurities, and the lignin is denatured by the impurities and the quality thereof may lower; and the method requires acid-resistant reaction facilities.